In power plants, flue gases from fuel often contain gaseous sulphuric oxides (SOx) because of sulphur compounds contained in the fuel. These flue gases are scrubbed with a gas scrubber, i.e. an absorber, in which sulphur dioxide is removed with a scrubbing fluid by using an absorbent in the scrubbing fluid, which absorbent reacts with the sure phur dioxide. Limestone-based scrubbers are based on the fact that the sulphur oxides entrained in the flue gases react with calcium carbonate and form calcium sulphite, which is oxidized to calcium sulphate (gypsum) by the effect of oxygen contained in the flue gases and forced oxidation. After this, the gypsum can be separated from the fluid and the fluid can be recycled to the scrubber.
Limestone scrubbers are advantageous to use because of their low operating costs (cheap absorbent). However, problems have been caused by the detection of foaming and cumulation of the foam in the limestone scrubber in diesel power plants and in other power plants. Entrained in the flue gases of a diesel power plant are lubricant residues and soot which, together with bubbles of the oxidation air, easily form an oily foam onto the fluid surface of the scrubber. The foaming is also affected by the quality of the water used in the scrubbing fluid. Because of the recirculation of the scrubbing fluid, some foam is also continuously recycled to the scrubber because it will not separate in a gypsum separator. As a result, the amount of foam on the fluid surface of the scrubber increases.
In power industry, it is possible to use a conventional solution to the problem of foaming: the use of anti-foam agent. However, anti-foam agents are expensive and thereby increase the operating costs of the scrubber.
It is also known to remove foam from the scrubbing fluid in a special container after the scrubber. The foam can be removed, for example, in an excess tank of a hydrocyclone, to which the scrubbing fluid separated from the gypsum is led. In the tank, the foam separates as a phase of its own onto the surface of the fluid and is removed from the fluid circulation by physical methods without chemicals. This arrangement is functional as such, but in all situations it is not possible or economically reasonable to construct such an apparatus in a power plant.
Soot and lubricant residues entrained in with the flue gases of a diesel power plant mix in the gypsum to be removed from the flue gas scrubber, colouring the gypsum dark, wherein it cannot be utilized in the gypsum board industry. Furthermore, with the flue gas and the raw water, impurities dissolving in the scrubbing fluid also enter the process, and these impurities are removed from the scrubber both as moisture in the gypsum and as waste water. With the flue gases and the raw water, e.g. chlorides and fluorides enter the process, causing corrosion in the scrubber.